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The Western Tent Caterpillar in Issaquah Highlands

By April 24, 2014June 27th, 2014Front Page

“Your IHCA staff has been fielding questions from many concerned residents about this caterpillar. It is now moving off its host trees and shrubs as a normal part of its life cycle. If they are a nuisance you can try a broom or a five year old but remember they are a passing thing. While they can be messy or gross they are not especially destructive and they do not kill landscape trees. Our contractors and staff have already killed most reachable nests and further treatments will not be occurring as they have stopped feeding.” 5/21/2014 Update by Russ Ayers, Landscape Manager, Issaquah Highlands Community Assn.


The most common tent caterpillar in home landscapes is the western tent caterpillar. Please don’t confuse this with the notorious gypsy moth caterpillar as our native ‘tent cat’ is much less destructive. It typically has a dark body with white and orange or yellow markings and a bluish dashed line down the center of the back. Long whitish or yellowish hairs are found along the length of the body. Adult moths lay eggs in a foam-like mass around current-year twigs, where the caterpillars overwinter as tiny larvae inside their eggshells.

In spring and early summer, characteristic tents are made on the tips of branches or elsewhere in trees and some shrubs. Young caterpillars typically feed in large groups in the protection of the nests. Older caterpillars may feed in small groups or as individuals. Potentially they can partially or completely defoliate trees, causing some loss of vigor, but this is uncommon. The western tent caterpillar is famous for 2- to 3-year epidemic cycles on many kinds of trees, including alder, Prunus species (which includes many shrubs), ash and to a lesser extent Hawthorne. They almost disappear for several years following outbreaks.

Tent Caterpillar Management Options

Select Non-chemical Management Options as Your First Choice!!

Pick out and destroy the foamy-looking, grayish, 1/2″ egg cases during the fall and winter. These may be found in bands around twigs or in flattened masses on trunks. It’s a good test of your eyes, too – the first egg case might take a while to spot;


Several natural parasites and predators help control tent caterpillar populations. Avoid use of broad-spectrum insecticides which kill beneficial insects. Our community’s Integrated Pest Management plan prohibits the use of broad-spectrum insecticides because of their known damage to non-target species and general ineffectiveness as a control;

Strip or prune out and destroy nests and caterpillars as soon as noticed. This is best done in early morning or evening, when caterpillars are gathered in the nests. Dispose of nests with your yard waste. Nests should be collected as close to Friday morning as possible.


Pesticides are generally not recommended for a host of reasons including applicator safety, generally low success rate and attendant undesired consequences including damage to beneficial insect populations. The community association and its contractors have had some success with Bt (Bacillus thuringiensis), a bacteria toxic to caterpillars, however, it can only be used on nests which can be reached by available equipment. Higher nests are out of reach. No pesticides may be applied to common area or street trees except by licensed IHCA staff or contractors. Violations should be reported to Issaquah Police (call 911) or the Washington Department of Agriculture’s pesticide enforcement division at 1-877-301-4555.